There are a lot of advantages, such as self-luminosity, low driving voltage, high luminous efficiency, short response time, high definition and contrast, nearly 180 degree viewing angle, wide temperature range, flexible display and large area full color display in Organic Light Emitting Diode (OLED) display devices. It's a very promising display technology.
According the driving method, OLED can be cataloged into two types: passive driving and positive driving, (i.e. addressing directly or through TFT. The active driving is called Active Matrix(AM) type. Each light emitting unit is controlled and addressed by TFT in the AMOLED device. The pixel driving circuit including the light emitting unit and TFT addressing circuit is driven by OVSS through the traces.
With the progress of the display technology, AMOLED display devices are developed toward the large size and high resolution. Correspondingly, the larger size panels and a larger number of pixels will be required in the large size AMOLED display devices. However, the conductive resistance is larger while the wire is longer. Inevitably, it will cause an IR drop of the negative voltage on the wire. The negative voltage of th for each pixel driving circuit will be different because of the various resistance of the wire.
With reference to FIG. 1, the conventional AMOLED display device includes: a substrate 10, a subpixel arranged in a array on the substrate, a plurality of rows of OVSS traces 20 arranged on the substrate 10; each row of subpixels is electrically connected with one of the rows of OVSS traces 20 where a negative voltage OVSS applied on. The negative voltage OVSS is inputted into the OVSS traces 20 from the edges of substrate 10. Due to the IR drop, the negative voltage OVSS of the subpixel in the center area of the substrate 10 is larger than the subpixel in the edge of the substrate 10 when AMOLED is on the display mode.
With reference to FIG. 2, the AMOLED device includes a first thin film transistor T100, a second thin film transistor T200, a capacitor C300 and an organic light emitting diode D100. In the first thin film transistor T100, the gate of is connected with a scan signal SCAN, the source is connected with a data signal DATA and the drain is connected with the gate of the second thin film transistor T200. In the second thin film transistor T200, the drain is connected with the positive voltage OVDD, and the source is connected with the anode of the organic light emitting diode D100. The two edges of the capacitor C100 are respectively connected with the gate and the source of the second thin film transistor T200. The cathode of the organic light emitting diode D100 is connected with the corresponding OVSS trace 20 and the negative voltage OVSS is applied on it.
When the subpixel arranged in the center area of the substrate 10 receives an elevated negative voltage OVSS due to the IR drop, the voltage on the gate of the second thin film transistor T200, acting as the driving TFT, will be elevated as well due to the capacitor coupling effect. This will shift the breakpoint voltage. In the conventional technology, the driving TFE is in the saturation region during displaying. This make the current passing through the driving TFT and the organic light emitting diode D100 steady. To lower the power consumption of driving the voltage on the drain of the driving TFT can be lowered (i.e. lowering down the positive voltage OVDD). In the conventional method, the impact on negative voltage due to the IR drop is not considered and it only based on the highest grayscale value or the highest data signal voltage to adjust the positive voltage OVDD. However, the driving TFT of the subpixel is still on the linear region. This incurs the bed uniformity in display.